A typical hydraulic system for a backhoe loader swing function is shown in FIG. 1. The manually operated direction control valve with pressure and tank connections is used to provide flow into one of two cylinders which function to oscillate a boom, and out of the opposite cylinder. A hydraulic hose or tube connects the piston rods of the cylinders such that when one cylinder extends, the other retracts. The rods of these cylinders are mechanically connected to the boom and control the rotation or swing function of the boom and any bucket attached thereto. With this system, the directional control valve is activated to start and maintain swing motion, and is centered, thus blocking all flow, to stop the swing motion. When this valve is centered, inertia of the moving boom and the bucket mass causes continued motion rather than an instantaneous stop. This continued motion compresses the fluid (especially any entrained air) in the cylinder that the boom and bucket are moving towards, thus resulting in a very high pressure in that cylinder. This high pressure stores energy much like a spring and creates a force that then causes the boom and the bucket to then move in the opposite direction, where the same phenomenon occurs. This oscillating motion eventually decays, but it delays the operator from continuing with digging or other work functions until the boom and the bucket stop. This also inhibits precise position control.
It is therefore a principal object of this invention to provide a swing cylinder oscillation control circuit and valve for oscillating booms which permits smooth deceleration at any stopping point in the swinging of the boom by sensing high pressure in one of the two cylinders that pivot or oscillate the boom.
A further object of this invention is to provide a swing cylinder oscillation control circuit and valve for oscillating booms which will permit an instantaneous stop of the oscillation of the boom by overcoming the inertia of the moving boom.
These and other objects will be apparent to those skilled in the art.
A swing cylinder oscillation control circuit and valve for oscillating booms has a hydraulic circuit of four valves incorporated into a typical boom control system. A shuttle valve is used to connect the higher pressure of one of the cylinders used to oscillate the boom to the inlet of a variable pressure check valve. Two standard check valves then connect the lower pressure of one or the other of the cylinders to the outlet of a variable pressure check valve. The variable pressure check valve is set to be closed at normal operating pressures and will open only when the pressure difference between the cylinders exceeds a set value, e.g., when the boom and the bucket swing momentum creates a high pressure in one cylinder. The variable pressure check valve opens at one pressure differential commonly known as the xe2x80x9ccrack pressurexe2x80x9d, and closes at another pressure differential, commonly known as the xe2x80x9cre-seat pressurexe2x80x9d. The crack and re-seat pressures are controlled to be adjustable and to be independent of each other. This valve then functions in the system by opening a flow connection between the two cylinders as soon as any swing xe2x80x9covershootxe2x80x9d causes a high pressure difference. The amount of fluid to be transferred through this connection is controlled by accurately setting both the crack and re-seat pressure of the variable pressure check valve. Transferring the proper amount of fluid from the high pressure cylinder to the low pressure cylinder at the onset of any swing xe2x80x9covershoot,xe2x80x9d then results in a smooth end-of-motion for the boom and the bucket swing, and provides precise position control.
A variable pressure check valve consists of a poppet with a sealing seat which slides in a bore; a piston which slides in a separate larger bore; a spring between the poppet and piston; a sleeve and retainer assembly containing the valve; and internal passages which interconnect these components. In normal operation, the spring holds the poppet against the seat, blocking flow from the inlet to the outlet. Inlet pressure is communicated through an orifice in the poppet to the top of the piston, and outlet pressure is communicated through a hole in the poppet to the bottom of the piston. The difference between higher inlet and lower outlet pressure creates a force on the piston holding it in the position shown and maintaining the spring force on the poppet. When the inlet-to-outlet pressure difference exceeds a pre-set value control by the poppet seat area and installed spring force, the poppet lifts creating a connection from the inlet to the outlet. This will cause the inlet and outlet pressures to begin to equalize. As this pressure difference decreases, the force on the piston decreases and the spring, normally in compression, will tend to cause the piston to lift. As the piston lifts, the spring compression is reduced therefore lowering the spring force that acts on the poppet. Since the spring force and the poppet seat area control the inlet-to-outlet pressure difference, this reduced force results in a reduced pressure difference or a re-seat pressure, that causes the valve to close. The opening and closing pressure are therefore determined by the poppet seat area; the spring strength and stiffness; and the difference between the poppet and piston strokes which can be controlled accurately and independently of each other.